1. Field of the Invention
The present invention relates to a process for the production of metal nanoparticles wherein a metal compound is reduced by a polyol in the presence of a substance that is capable of being adsorbed on the nanoparticles and wherein the precipitation of the formed nanoparticles by a nanoparticle-precipitating liquid is improved by the addition of a protic solvent.
2. Discussion of Background Information
The production of metal particles by the polyol process is known from, e.g., U.S. Pat. No. 4,539,041 to Figlarz et al., the entire disclosure whereof is expressly incorporated by reference herein. In the polyol process, a metal compound is reduced at an elevated temperature by a polyol to afford the corresponding metal in the form of particles (usually in the micron and nanometer size range). A number of metal compounds and in particular, a number of transition metal compounds can be converted to metal particles by this process. In a typical procedure, a solid metal compound is suspended in a polyol and the suspension is heated until the reduction of the metal compound is substantially complete. Thereafter, the formed particles are isolated by separating them from the liquid phase, e.g., by centrifugation.
A modification of this method is described in, e.g., P.-Y. Silvert et al., “Preparation of colloidal silver dispersions by the polyol process” Part 1—Synthesis and characterization, J. Mater. Chem., 1996, 6(4), 573-577; and Part 2—Mechanism of particle formation, J. Mater. Chem., 1997, 7(2), 293-299. According to the Silvert et al. articles, the entire disclosures whereof are expressly incorporated by reference herein, the polyol process is carried out in the presence of a polymer, i.e., polyvinylpyrrolidone (PVP). In a typical experiment, 1-25 g of PVP was dissolved at room temperature in 75 ml of ethylene glycol and 50-3,200 mg of silver nitrate was added to this solution. The resultant suspension was stirred until the silver nitrate had dissolved completely, whereafter the system was heated until the reaction was complete. It is reported that after cooling with tap water, the silver particles could be easily separated from the ethylene glycol by adding a “large amount of acetone” to the resultant colloidal dispersion, followed by centrifugation.
While the results reported by Silvert et al. are desirable, the present inventors have found that when a corresponding process is scaled up and conducted with a significantly larger amount of metal compound than that described by Silvert et al., even the addition a large amount of a particle-precipitating liquid such as acetone does not afford a satisfactory separation of the metal nanoparticles from the polyol or the liquid phase of the reaction mixture, respectively. In particular, frequently the separation requires unacceptably long periods of time or does not take place at all. Accordingly, it would be advantageous to have available a process of the type described by Silvert et al. which affords satisfactory results in terms of the separation of the nanoparticles from the liquid phase even when it is conducted on a larger scale than that reported by Silvert et al.